Variable reactance element



Dec. '10, 1968 9|, VALLESE I 3,416,105

VARIABLE REACTANCE ELEMENT Filed April 21, 1966 Z-Sheets-Sheet 1INVENTOR. LUC/O M. VALLESE xwm/i'wmk ATTORNEY 06. 10, 1968 L. y. VALLESE3,416,105

VARIABLE REACTANCE ELEMENT Filed April 21, 1966 2 Sheets-Sheet 2 I f CFIG. 2

INVENTOR. LUC/O M. VALCES B ATTORNEY United States Patent 3,416,105VARIABLE REACTANCE ELEMENT Lucio M. Vallese, Glen Ridge, N.J., assignorto International Telephone and Telegraph Corporation, a corporation ofDelaware Filed Apr. 21, 1966, Ser. No. 544,199

7 Claims. (Cl. 333-80) ABSTRACT OF THE DISCLOSURE This is a nonlinearreactance element having a variable resistance element in series withthe secondary of a transformer whereby variations in the resistanceelement produces variations of inductive reactance at the primaryterminals of the transformer. Negative resistance means are electricallycoupled to the primary winding in order to reduce the effectivereflected resistance from the secondary to the primary terminals.

This invention relates to variable reactance circuit elements and moreparticularly to an improved electrically variable inductive reactancecircuit element.

Electrically controllable inductive reactance circuit elements arewidely utilized in frequency modulation transmission equipment,parametric amplifiers, tunable filters and parametron logic devices.These elements have heretofore been realized by electromechanical means,reactance tube techniques and ferrite devices.

Electromechanical techniques have been employed to change the mechanicalparameters associated with an inductor in response to suitableelectrical control signals. Such techniques have been limited to lowfrequency applications and been relatively unreliable, requiringrelatively large amounts of power and occupying considerable space andweight.

Reactance tube techniques, while widely used in frequency modulators,require frequency sensitive phase shift networks to insure that thevoltage and current applied to the reactance tube element remain 90 outof phase throughout the range of frequencies over which the device is tooperate, and are thus unsuitable for broadband applications.

Ferrite inductors utilize the nonlinear B-H characteristic of theferrite material to obtain a variation of incremental inductance as thebias current through the inductor surrounding the ferrite is varied.Such ferrite inductors;-are useful at only relatively low frequencies (afew megacycles) and are subject to inherent saturation effects.

Accordingly, an object of the present invention is to provide a variableinductive reactance circuit element which is capable of operating over abroad frequency band.

Another object of the invention is to provide an inductive reactancecircuit element capable of operating at relatively high frequency andrelatively insensitive to saturation effects.

These and other objects which will become apparent by reference to thefollowing detailed description, the appended claims, and theaccompanying drawings are realized by varying the resistive load in thesecondary of a transformer in such a manner that the inductive reactanceseen looking into the transformer primary varies in synchronism with theload resistance variation.

The invention will be more readily understood by reference to thefollowing detailed description taken in conjunction with theaccompanying drawing in which:

FIGS. 1a and 1b show the general principle of operation of an inductivereactance circuit element according to the invention.

3,416,105 Patented Dec. 10, 1968 FIG. 2 shows a particular embodiment ofa circuit element according to the invention.

FIGS. 3a, 3b and 3c show improved embodiments of inductive reactancecircuit elements according to the invention.

FIG. 4 shows a diagram useful for purposes of explaining the invention.

FIG. 5 shows a parametron logic element incorporating an inductivereactance circuit element according to the invention.

FIG. 1a shows a transformer of the linear (i.e., having a linear B-Hcharacteristic core) type consisting of first and second inductanceelements 3 and 4 which have mutual inductance therebetween. A variableresistance element 5 is connected to the secondary inductance element 4.The equivalent circuit for the network of FIG. 1a is shown in FIG. 1b,where R and L represent the resistance and reactive components of firstinductance element 3, and R and C representthe resistive and reactivecomponents of driving point impedance (i.e., impedance seen looking intoterminals 1 and 2) reflected into the primary circuit by mutual couplingbetween the inductances. It

maybe readily shown that 130241021322y cl Ro -I' where w equals theoperating angular frequency.

=the mutual inductance between the inductance elements.

R =the loop resistance in the transformer secondary circuit, i.e., thesum of the variable resistance 5 and the resistance of inductanceelement 4.

L the self-inductance of the inductance element 4.

X =the (capacitive) reactance reflected into the primary circuit.

By differentiating the expression for R with respect to Therefore if avalue of the variable resistance element 5 is chosen such that the totalsecondary loop resistance is equal to the self-reactance of inductanceelement 4, small variations of the resistance element 5 from this valuewill result in corresponding variation of the reflected capacitivereactance X while having substantially no effect on the reflectedresistance R. It may be readily shown that the capacitive reactance X isalways less than the self-reactance of inductance element 3, so that thereactive component of the driving point impedance seen looking intoterminals 1 and 2 is always inductive.

Since under the foregoing conditions R is substantially equal to X thequality factor of the variable inductive reactance circuit of FIG. 1awill be quite low, although sufficiently high for many practicalapplications. This quality factor may be improved by provision of asuitable negative resistance to effectively cancel the reflectedresistive component R or by suitable modification of the transformerwindings 3 and 4 to efiect such cancellation.

FIG. 2 shows an inductive reactance circuit element according to theinvention. The transistor Q serves as the variable resistance element,and is controlled by the voltage source V The resistor 6 is preferablychosen so that the transistor Q effects variation of the secondary loopresistance about a mean value substantially equal to the self-inductanceof inductance element 4. The battery B supplies proper bias for thetransistor Q and should be chosen such that the current bias applied toinductance element 4 does not produce excessive heating or undesirednonlinearity effects. It is evident from the foregoing discussion thatif the voltage V is varied at a frequency w the inductive reactance seenlooking into terminals 1 and 2 will vary accordingly, while the apparentresistance seen looking into these terminals will be substantiallyconstant.

FIG. 4 plots the reflected resistive and reactance components as afunction of the secondary loop resistance R It can be seen that at thepoint Where R =wL the slope of the R curve is substantially zero whilethe X curve has substantial slope and is approximately linear in thisregion.

FIG. 3a shows the circuit of FIG. 2 with added provision of a voltagesource V in the primary circuit. If V is synchronized with V therelative phase of V may be chosen so as to effect cancellation of thereflected resistance R; when utilized in this manner, the source V actsessentially as a negative resistance element equal and opposite to thereflected resistance R.

FIG. 3b shows an alternative negative resistance element consisting ofthe transistor Q, the resistor 7 and the voltage source V". The voltageV could be obtained by coupling a suitable phase shift network betweenthe secondary voltage source V and the base of the transistor Q.

FIG. 30 shows an alternative technique for compensating the reflectedresistance R. In this approach, each of the inductance elements 3 and 4is divided into two operating portions 8 and 9 and 10 and 11respectively. The sign of the mutual inductance between portions 8 and10 is opposite to that of the mutual inductance between portions 9 and11. The circuit parameters are selected so that the real components ofthe reflected impedances cancel while the imaginary components add.

FIG. 5 shows a parametron logic component employing the inductivereactance circuit element of the invention. The transformer consistingof windings 14 and 15 serves to couple the driving signal applied toterminals 12 and 13 to the inductive reactance element terminals 1 and2. The capacitor C serves to resonate the inductive reactance circuitelement at a frequency equal to one half the drive frequency. Thissubharmonic of the drive frequency is coupled to other parametroncomponents in the logic network via output terminals 17 and 18 throughisolating resistor 16. The inductance of the inductive reactance circuitelement (FIG. 2) is varied at a frequency equal to the drivingfrequency. The inductive reactance element thus serves to produce thedesired subharmonic by parametric coupling of the drive signal, thesubharmonic phase being synchronized with the phase of the drive signal.The manner in which parametrons are utilized ip logic networks is wellknown in the art, the operation of the circuit of FIG. 5 as viewed atterminals 1 and 2 being similar to parametron circuitry employing othertypes of variable inductive reactance elements.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

What is claimed is:

1. A variable reactance circuit element comprising:

first and second inductance elements having mutual inductancetherebetween;

a variable resistance element electrically coupled to said secondinductance element, said resistance element together with said secondinductance element providing an impedance having a resistive andinductive component which is reflected from said second inductanceelement and is exhibited across the terminals of said first inductanceelement so that a variation in said resistance element causes a changein said inductance component of said reflected impedance across theterminals of said first inductance element; and

negative resistance means electrically coupled to said first inductanceelement causing said resistive component of said reflected impedance tobe reduced.

2. A circuit element according to claim 1 wherein the resistance of saidresistance element is varied about a mean value substantially equal tothe magnitude of the self-reactance of said second inductance element.

3. A circuit element according to claim 2, wherein the variation of saidresistance is small in comparison with said mean value.

4. A circuit element according to claim 1, wherein said resistanceelement includes:

a semiconductor device having input and output terminals; and

drive means operatively connected to said input terminals for varyingthe current through said output terrninals,

said current being substantially in phase with the voltage across saidoutput terminals.

5. A circuit arrangement according to claim 4, wherein said negativeresistance means includes a source of voltage synchronized with saiddrive means of said resistance element.

6. A circuit element according to claim 4, wherein each of saidinductance elements comprises a plurality of operating portions, eachoperating portion of said first inductance element being inductivelycoupled to a corresponding operating portion of said second inductanceelement, said operating portions being arranged so as to reduce saidresistive component.

7. A circuit element according to claim 4, wherein said negativeresistance means includes a transistor having a base, emitter andcollector, a source of voltage coupled to said base, and a resistorconnected across said collector and emitter, said resistor beingconnected in series with said first inductance element.

References Cited UNITED STATES PATENTS 2,570,939 10/1951 Goodrich.2,928,036 3/1960 Walker. 3,168,710 2/1965 Schultz.

HERMAN KARL SAALBACH, Primary Examiner.

PAUL GENSLER, Assistant Examiner.

US. Cl. X.R.

